Gray cast iron for cylinder liner and method for manufacturing cylinder liner using the same

ABSTRACT

A gray cast iron for a cylinder liner is provided that maintains fatigue strength and thermal shock property by adjusting component contents. The gray cast iron includes carbon (C) in an amount of about 3.2 to 3.7 weight % (wt %); silicon (Si) in an amount of about 2.0 to 2.8 wt %; manganese (Mn) in an amount of about 0.50 to 1.0 wt %; phosphorus (P) in an amount of about 0.20 wt % or less, and greater than about 0 wt %; sulfur (S) in an amount of about 0.10 wt % or less, and greater than about 0 wt %; chromium (Cr) in an amount of about 0.50 wt % or less, and greater than about 0 wt %; copper (Cu) in an amount of about 0.20 to 0.80 wt %; molybdenum (Mo) in an amount of about 0.10 to 0.40 wt %; and the balance of iron (Fe), based on the total weight of the gray cast iron.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2014-0139790, filed Oct. 16, 2014, the entire contents of which is incorporated herein for all purposes by this reference.

TECHNICAL FIELD

The present invention relates to a gray cast iron for a cylinder liner and a method for manufacturing the cylinder liner using the gray cast iron. In particular, the gray cast iron for a cylinder liner may maintain fatigue strength and thermal shock property by adjusting contents of a molten steel composition for manufacturing the gray iron cast.

BACKGROUND OF THE INVENTION

Since a gray cast iron costs less and has excellent performance, it has been widely used for a cylinder block, a cylinder head, and a cylinder liner of an engine for a vehicle. Those component parts of the vehicle may be used to compromise material properties such as tensile strength, fatigue strength, and the like of a material and productivity such as cost, workability, and the like when being used and manufactured, and thus, various components of the gray cast iron may be combined at a suitable level.

Recently, due to an increase in an output of the vehicle, reinforcement of environment regulation, and the like, loads across the block, the head, and the liner have been increased. Consequently, a need for a novel gray cast iron that may improve the material property and maintain the productivity has increased.

Meanwhile, the cylinder liner of the engine may be a key part inserted into the cylinder block and configuring a combustion chamber. A cylinder liner in the related art has been mainly formed by using a general gray cast iron material having a tensile strength of about 250 MPa. Although the above-mentioned gray cast iron may be suitable for a general gasoline engine in which combustion pressure and load are low, deterioration in durability may be caused when used for a recently developed high output engine or diesel engine.

To overcome the above-mentioned problem, for example, a 400 MPa grade cast iron has been manufactured by adding alloy elements to the conventional 250 MPa grade cast iron and partially used for a large diesel engine, or the like. However, manufacturing cost thereof is high compared to a conventional material. In addition, according to a conventional art, a technology for a method for manufacturing a high strength gray cast iron utilizing a high manganese steel scrap has been proposed. However, although the high strength may be maintained while production cost is reduced utilizing the scrap, an expensive alloy element may be added thereby increasing manufacturing cost.

The above information disclosed in this background section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY OF THE INVENTION

In preferred aspects, the present invention provides a gray cast iron for a cylinder liner that can maintain fatigue strength and thermal shock property substantially by adjusting contents of alloy components. In particular, the alloy components in the gray cast iron composition may stabilize pearlite and improve hardness of a matrix structure without adding expensive alloy elements. Further, the present invention provides a method for manufacturing the cylinder liner using the gray cast iron composition.

According to an exemplary embodiment of the present invention, a gray cast iron is provided for a cylinder liner. The gray cast iron may comprise: carbon (C) in an amount of about 3.2 to 3.7 weight % (wt %); silicon (Si) in an amount of about 2.0 to 2.8 wt %; manganese (Mn) in an amount of about 0.50 to 1.0 wt %; phosphorus (P) in an amount of about 0.20 wt % or less, and greater than about 0 wt %; sulfur (S) in an amount of about 0.10 wt % or less, and greater than about 0 wt %; chromium (Cr) in an amount of about 0.50 wt % or less, and greater than about 0 wt %; copper (Cu) in an amount of about 0.20 to 0.80 wt %; molybdenum (Mo) in an amount of about 0.10 to 0.40 wt %; and the balance of iron (Fe), based on the total weight of the gray cast iron. In particular, the gray cast iron composition may satisfy the following Equation 1.

0.8≦Cu+1.5Cr+1.2Mo≦1.5  [Equation 1]

In the Equation 1, Cu, Cr, and Mo respectively mean the contents in wt % of Cu, Cr, and Mo components.

The gray cast iron may have a tensile strength of about 300 MPa or greater. The gray cast iron may have a fatigue strength of about 140 MPa or greater.

It is also provided that the gray cast iron may consist of, or consist essentially of the above-mentioned components in its composition. For example, the gray case iron as described herein may consist or consist essentially of carbon (C) in an amount of about 3.2 to 3.7 weight % (wt %); silicon (Si) in an amount of about 2.0 to 2.8 wt %; manganese (Mn) in an amount of about 0.50 to 1.0 wt %; phosphorus (P) in an amount of about 0.20 wt % or less, and greater than about 0 wt %; sulfur (S) in an amount of about 0.10 wt % or less, and greater than about 0 wt %; chromium (Cr) in an amount of about 0.50 wt % or less, and greater than about 0 wt %; copper (Cu) in an amount of about 0.20 to 0.80 wt %; molybdenum (Mo) in an amount of about 0.10 to 0.40 wt %; and the balance of iron (Fe), based on the total weight of the gray cast iron

According to another exemplary embodiment of the present invention, a method is provided for manufacturing a cylinder liner. The method may include: manufacturing a molten steel that comprises carbon (C) in an amount of about 3.2 to 3.7 weight % (wt %); silicon (Si) in an amount of about 2.0 to 2.8 wt %; manganese (Mn) in an amount of about 0.50 to 1.0 wt %; phosphorus (P) in an amount of about 0.20 wt % or less, and greater than about 0 wt %; sulfur (S) in an amount of about 0.10 wt % or less, and greater than about 0 wt %; chromium (Cr) in an amount of about 0.50 wt % or less, and greater than about 0 wt %; copper (Cu) in an amount of about 0.20 to 0.80 wt %; molybdenum (Mo) in an amount of about 0.10 to 0.40 wt %; and the balance of iron (Fe), based on the total weight of the gray cast iron; and casting the molten steel by centrifugal casting. In particular, the components in the molten steel composition may satisfy the Equation 1 as described above.

After the centrifugal casting of the molten steel, the cylinder liner may have a tensile strength of about 300 MPa or greater. After the centrifugal casting of the molten steel, the cylinder liner may have a fatigue strength of about 140 MPa or greater.

Further provided is a vehicle part that may be manufactured from the gray cast iron or by the method as described herein.

Other aspects of the invention are also disclosed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows exemplary micro-structure photographs of an exemplary gray cast iron for a cylinder liner according to an exemplary embodiment of the present invention; and

FIG. 2 shows micro-structure photographs of an exemplary gray cast iron according to Comparative Examples.

DETAILED DESCRIPTION

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term “about.”

Hereinafter, exemplary embodiments of the present invention will be described in more detail with reference to the accompanying drawings. However, the present invention is not limited to exemplary embodiments disclosed below, but will be implemented in various forms. The exemplary embodiments of the present invention make disclosure of the present invention thorough and are provided so that those skilled in the art can easily understand the scope of the present invention.

In an exemplary embodiment, the present invention provides a gray cast iron. The gray cast iron may comprise: carbon (C) in an amount of about 3.2 to 3.7 weight % (wt %); silicon (Si) in an amount of about 2.0 to 2.8 wt %; manganese (Mn) in an amount of about 0.50 to 1.0 wt %; phosphorus (P) in an amount of about 0.20 wt % or less, and greater than about 0 wt %; sulfur (S) in an amount of about 0.10 wt % or less, and greater than about 0 wt %; chromium (Cr) in an amount of about 0.50 wt % or less, and greater than about 0 wt %; copper (Cu) in an amount of about 0.20 to 0.80 wt %; molybdenum (Mo) in an amount of about 0.10 to 0.40 wt %; and the balance of iron (Fe), based on the total weight of the gray cast iron.

The amount of carbon (C) may be in a range of about 3.2 wt % to 3.7 wt %, based on the total weight of the gray cast iron. Carbon, as used herein, may be an essential element for improving hardness and wear resistance by forming flake graphite and reducing carbide while being coagulated. For example, a carbon equivalent (C_(eq)=C+1/3Si) may be calculated by the contents of carbon and contents of silicon, and when the carbon equivalent is close to (e.g., about) a process point of about 4.3 wt %, a melting point of a molten metal may be reduced and fluidity may be improved. However, when the amount of carbon equivalent is greater than the process point, a crystallization amount of graphite having low hardness and strength may be increased, thereby decreasing strength of a cast iron. In addition when the amount of carbon equivalent is less than the processed point, fluidity of the molten metal may be decreased, thereby causing cast defect, and also, an amount of flake graphite may be decreased, thereby decreasing lubrication property. Accordingly, the carbon content may be in a range of about 3.2 to about 3.7 wt %. When the carbon content is less than the above-mentioned range, for example, less than about 3.2 wt %, the casting defect may be caused, and when the carbon content is greater than the above-mentioned range, for example, greater than about 3.7 wt %, strength and fatigue life may be decreased.

The amount of silicon (Si) may be in a range of about 2.0 wt % to about 2.8 wt %, based on the total weight of the gray cast iron. Silicon, as used herein, may be one of the main elements that determine carbon equivalent together with carbon, and may be an element which contributes to form graphite and improve oxidation-resistant. Since formation of graphite may be promoted as the silicon content in the carbon equivalent which is calculated the amount of carbon and the amount of silicon is increased, the crystallization amount of graphite may be increased even at the same carbon amount. In addition, silicon may increase heat-resisting property and improve thermal shock property of a material by forming Si-based oxidation-resistant film in an oxidation environment. Accordingly, castability and oxidation resistant property may be suitably obtained by limiting the contents of silicon in a range of about 2.0 to 2.8 wt %.

The amount of manganese (Mn) may be in a range of about 0.50 wt % to 1.0 wt %, based on the total weight of the gray cast iron. Manganese, as used herein, may be an element which is combined with sulfur (S) to form MnS, and the MnS may be a lubricating phase and stabilize carbide to improve strength. However, when the manganese content is less than the proposed range, for example, less than about 0.5 wt %, MnS may be insufficiently formed, and thus, a desired level of lubricity may not be secured. When the manganese content is greater than the proposed range, for example, greater than about 1.0 wt %, crystallization of graphite may be interrupted and the formation of coarse carbide may be promoted, and thus, friction property may be reduced. Accordingly, the manganese content may be contained in a range of about 0.50 to 1.0 wt %, and as a result, a suitable amount of lubricating phase of MnS may be formed, thereby improving friction property.

The amount of phosphorus (P) may be in a range of about 0.20 wt % or less, and greater than about 0 wt %, based on the total weight of the gray cast iron. Although phosphorus may be considered as impurity, when a substantial amount of the phosphorus is added, the phosphorous may form steadite phase of high hardness having a composition of Fe₃P in a matrix structure to improve wear-resistant. When a suitable amount of steadite phase is substantially uniformly distributed in the matrix structure, it may provide an advantageous effect. However, since substantial coarse steadite phase decreases workability, the content of the phosphorous may be limited within about 0.2 wt %.

The amount of chromium (Cr) may be in a range of about 0.50 wt % or less, and greater than about 0 wt %), based on the total weight of the gray cast iron. The amount of copper (Cu) may be in a range of about 0.20 wt % to 0.80 wt %, and the amount of molybdenum (Mo) may be in a range of about 0.10 wt % to 0.40 wt %. Copper (Cu), chromium (Cr), and molybdenum (Mo), as used herein, may be elements that stabilize pearlite and improve hardness of the matrix structure, and thus, increase hardness of a product, for example, by solid solution and deposition strengthen effect. However, when amounts thereof are added greater than the predetermined amounts, workability may deteriorate.

Particularly, copper may be one of the most important element of stabilizing pearlite, and adding a suitable amount of copper may be critical. In addition, since molybdenum is partially solid-dissolved in the matrix structure to allow soft resistance of the matrix structure to be increased at a substantially high temperature, durability may be improved when the produced parts are exposed to thermal shock due to the suitable amount of the molybdenum. Accordingly, an optimal ratio of the contents of Cr, Cu, and Mo has been considered through many tests, and as a result, Cr may be optimally limited to the contents of about 0.5 wt % at maximum, Cu may be optimally limited to the contents in a range of about 0.2 to 0.8 wt %, and Mo may be optimally limited to the contents in a range of about 0.1 to 0.4 wt %.

In an exemplary embodiment of the present invention, a typical centrifugal casting method may be used for the molten steel that may have the above-mentioned composition in order to manufacture the cylinder liner having substantially improved fatigue strength and thermal shock property. Particularly, when the contents of each component of the molten steel are adjusted, the contents in wt % of Cu, Cr, and Mo may be adjusted to maintain tensile strength of cylinder liner of about 300 MPa or greater and maintain fatigue strength of about 140 MPa or greater. For example, the following Equation 1 may be satisfied by the composition.

0.8≦Cu+1.5Cr+1.2Mo≦1.5  [Equation 1]

In Equation 1, a value which is calculated by Cu+1.5Cr+1.2Mo is referred to as “A” for convenience of explanation unless otherwise is indicated. In Equation 1, Cu, Cr, and Mo respectively mean the contents in wt % of Cu, Cr and Mo components.

The A, which is the value calculated by Equation 1, may be determined by the contents of Cu, Cr, and Mo. Cu, Cr, and Mo may be solid-dissolved in the matrix structure and parts thereof may be element for forming micro-carbide. When the value of A satisfies a range of about 0.8 to 1.5, the decrease in workability may be minimized and the targeted material properties, such as tensile strength and fatigue strength, may be suitably obtained. When the value of A is greater than the proposed range, for example greater than about 1.5, coarse harmful carbide may be formed, and when the value of A is less than the proposed range, for example, less than about 0.8, the targeted material properties may not be achieved.

EXAMPLES

Hereinafter, the present invention will be described with reference to Comparative Examples and Inventive Examples.

A final product was produced according to a production condition of a cylinder liner which was commercially produced, samples were manufactured from the final product. Hardness, tensile strength, fatigue strength, thermal fatigue life, and workability were then measured.

As shown in Table 1, in Inventive Examples 1 and 2, and Comparative Examples 1 to 5, molten steel having the adjusted contents of components were prepared, and each cylinder liner was then manufactured by the centrifugal casting method. In addition, the samples were manufactured by the manufactured cylinder liner.

TABLE 1 A C Si Mn P S Cr Cu Mo Fe (Cu + 1.5Cr + Classification (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) 1.2Mo) Remark Inventive 3.4 2.5 0.8 0.05 0.05 0.22 0.44 0.15 Rem. 0.95 Example 1 Inventive 3.5 2.4 0.75 0.08 0.06 0.4 0.54 0.25 Rem. 1.44 Example 2 Comparative 3.5 2.2 0.7 0.05 0.05 0.05 0 0 Rem. 0.075 Existing Example 1 Material Comparative 3.3 2.6 0.8 0.16 0.05 0.6 0.28 0.05 Rem. 1.24 Similar Example 2 Component System Comparative 3.4 2.5 0.83 0.08 0.04 0.4 0.7 0.25 Rem. 1.6 Similar Example 3 Component System Comparative 3.4 2.7 0.8 0.1 0.06 0.15 0.3 0.15 Rem. 0.705 Similar Example 4 Component System Comparative 3.6 2.2 0.75 0.06 0.05 0.35 0.7 0.05 Rem. 1.285 Similar Example 5 Component System

Meanwhile, hardness was measured using Brinell machine after the manufactured sample was flattened, tensile strength was measured by machining the cylinder liner with KS D0801 8A test piece, and fatigue strength was defined as a load when fatigue life was 1 million cycle or greater after a sample having an unnotched shape was manufactured and rotation bending fatigue test was performed. In addition, thermal fatigue life was performed by repeatedly performing heating and cooling at a temperature interval of about 100 to 350° C. at a condition of a constraint ratio of about 40% under thermal mechanical fatigue condition, and workability was calculated based on tool life when performing a rough grinding of an inner diameter of the liner. The results from measured values were shown in Table 2. In addition, micro-structures of Inventive Examples and Comparative Examples were photographed using an optical in microscope at 100 magnification and were shown in FIGS. 1 and 2.

TABLE 2 Tensile Fatigue Thermal Hardness Strength Strength Fatigue Life Work- Classification (HRB) (MPa) (MPa) (cycles) ability Inventive 99 310 141 630 0.98 Example 1 Inventive 102 321 150 598 0.96 Example 2 Comparative 93 260 98 135 1 Example 1 Comparative 103 285 106 167 0.88 Example 2 Comparative 105 317 139 415 0.73 Example 3 Comparative 98 291 123 243 0.95 Example 4 Comparative 100 329 145 215 0.84 Example 5

As shown in Table 2, it may be appreciated that in Inventive Examples 1 and 2, mechanical properties were improved. For example, the hardness was improved by about 5 to 10%, the tensile strength was improved by about 20%, and the fatigue strength was improved by about 40 to 50% as compared to Comparative Example 1 which was a conventional material. In addition, it may be appreciated that thermal fatigue life was significantly increased by about 3 times or greater and workability maintained at a level of 90% or greater. In addition, FIG. 1 shows exemplary micro-structure photographs of an exemplary gray cast iron for a cylinder liner manufactured according to an exemplary embodiment of the present invention, and it may be appreciated from the micro-structure photographs of Inventive Examples 1 and 2 that micro-carbide was uniformly formed on a pearlite matrix structure.

In addition, comparing Inventive Examples 1 and 2 with the results of Comparative Examples 2 to 5, although there were cases in which Comparative Examples exhibit improved hardness, tensile strength, and fatigue strength, and the like, it may be appreciated in terms of a combination of the respective properties that Comparative Examples 2 to 5 were less than the hardness, tensile strength, and fatigue strength, and the like of Inventive Examples 1 and 2.

Meanwhile, FIG. 2 shows exemplary micro-structure photographs of a conventional gray cast iron according to Comparative Examples. For example, Comparative Example 1 was a general gray cast iron material producing a conventional 250 MPa grade cylinder liner and had a hardness of about 93(HRB), tensile strength of about 260 Mpa, fatigue strength of about 98 Mpa, and thermal fatigue life of about 135 cycle, when a reference (100%) of workability was defined as tool life upon rough grinding of Comparative Example 1. In addition, as shown in FIG. 2, in Comparative Example 1, flake graphite was formed on the pearlite matrix structure.

Meanwhile, Comparative Example 2 had the value of A, which is a summed range of the contents of Cu, Cr, and Mo, within the proposed values of the present invention, but the content of each element was out of the proposed range of the present invention. For example, the Cr content was greater than the proposed value, such that a great quantity of coarse Cr based carbides was formed as shown in FIG. 2. As a result, hardness was improved as compared to Inventive Example 1, but other material properties were not significantly improved.

In addition, Comparative Example 3 included each element that satisfied the proposed range of the present invention, but the value of A which is the summed range of the contents of Cu, Cr, and Mo, was greater than the proposed value, for example, greater than about 1.5. As shown in FIG. 2, in Comparative Example 3, workability was significantly decreased while hardness was significantly increased due to excessive formation of micro-carbide.

Further, Comparative Example included each element that satisfied the proposed range of the present invention, but the value of A, which is the summed range of the contents of Cu, Cr, and Mo, was not obtained with in the proposed range of the present invention. As shown in FIG. 2, material property was improved as compared to Comparative Example 1 since micro-carbide was not formed at a desired level, but fatigue strength and thermal fatigue life did not satisfy a desired level as compared to Inventive Examples 1 and 2.

Additionally, Comparative Example 5 had the value of A, which is the summed range of the contents of Cu, Cr, and Mo, within the proposed values of the present invention, but the contents of Mo was not in the proposed range of the present invention. As shown in FIG. 2, the most material properties satisfied at the target level since micro-carbide was not formed at a desired level, but thermal fatigue life was significantly decreased as compared to Inventive Examples 1 and 2.

Therefore, to maintain or improve hardness, tensile strength, fatigue strength, thermal fatigue life, and workability at a desired level by forming the micro-carbide on the pearlite matrix structure suitably, the contents of Cu, Cr, and Mo may be controlled, such that the value of A obtained from the Equation 1 may be in a range from about 0.8 to 1.5.

According to various exemplary embodiments of the present invention, the 300 MPa grade gray cast iron material that may have an improved tensile strength by about 20% compared to the conventional 250 MPa grade gray cast iron may be manufactured by optimally adjusting the contents of the alloy components, which particularly may influence stabilization of the pearlite and improvement of hardness of the matrix structure among the components of the molten steel for manufacturing the gray cast iron.

As a result, the fatigue strength of about 140 MPa of the gray cast iron may be obtained which may be improved by about 40% compared to the conventional 100 MPa and the gray cast iron capable of being used even in a high temperature/high pressure combustion chamber may also be manufactured by substantially improving the thermal shock property. Moreover, since workability may maintain a level similar to the conventional material, substantial advantages may be expected in terms of production.

Although the present invention has been described with reference to the accompany drawings and various exemplary embodiments of the present invention, the present invention is not limited thereto and is defined in the following claims. Accordingly, those skilled in the art may variously modify and alter the present invention without departing from the spirit and the scope defined by the following claims. 

What is claimed is:
 1. A gray cast iron for a cylinder liner, comprising: carbon (C) in an amount of about 3.2 to 3.7 weight % (wt %) based on the total weight of the gray cast iron; silicon (Si) in an amount of about 2.0 to 2.8 wt % based on the total weight of the gray cast iron; manganese (Mn) in an amount of about 0.50 to 1.0 wt % based on the total weight of the gray cast iron; phosphorus (P) in an amount of about 0.20 wt % or less, and greater than about 0 wt % based on the total weight of the gray cast iron; sulfur (S) in an amount of about 0.10 wt % or less, and greater than about 0 wt % based on the total weight of the gray cast iron; chromium (Cr) in an amount of about 0.50 wt % or less, and greater than about 0 wt % based on the total weight of the gray cast iron; copper (Cu) in an amount of about 0.20 to 0.80 wt % based on the total weight of the gray cast iron; molybdenum (Mo) in an amount of about 0.10 to 0.40 wt % based on the total weight of the gray cast iron; and the balance of iron (Fe), wherein the gray cast iron satisfies the following Equation 1, 0.8≦Cu+1.5Cr+1.2Mo≦1.5  [Equation 1] where Cu, Cr, and Mo respectively mean the contents in wt % of Cu, Cr and Mo components.
 2. The gray cast iron of claim 1, wherein the gray cast iron has a tensile strength of about 300 MPa or greater.
 3. The gray cast iron of claim 1, wherein the gray cast iron has a fatigue strength of about 140 MPa or greater.
 4. The gray cast iron of claim 1, consisting essentially of: carbon (C) in an amount of about 3.2 to 3.7 weight % (wt %) based on the total weight of the gray cast iron; silicon (Si) in an amount of about 2.0 to 2.8 wt % based on the total weight of the gray cast iron; manganese (Mn) in an amount of about 0.50 to 1.0 wt % based on the total weight of the gray cast iron; phosphorus (P) in an amount of about 0.20 wt % or less, and greater than about 0 wt % based on the total weight of the gray cast iron; sulfur (S) in an amount of about 0.10 wt % or less, and greater than about 0 wt % based on the total weight of the gray cast iron; chromium (Cr) in an amount of about 0.50 wt % or less, and greater than about 0 wt % based on the total weight of the gray cast iron; copper (Cu) in an amount of about 0.20 to 0.80 wt % based on the total weight of the gray cast iron; molybdenum (Mo) in an amount of about 0.10 to 0.40 wt % based on the total weight of the gray cast iron; and the balance of iron (Fe).
 5. A method for manufacturing a cylinder liner, comprising: manufacturing a molten steel comprising carbon (C) in an amount of about 3.2 to 3.7 weight % (wt %); silicon (Si) in an amount of about 2.0 to 2.8 wt %; manganese (Mn) in an amount of about 0.50 to 1.0 wt %; phosphorus (P) in an amount of about 0.20 wt % or less, and greater than about 0 wt %; sulfur (S) in an amount of about 0.10 wt % or less, and greater than about 0 wt %; chromium (Cr) in an amount of about 0.50 wt % or less, and greater than about 0 wt %; copper (Cu) in an amount of about 0.20 to 0.80 wt %; molybdenum (Mo) in an amount of about 0.10 to 0.40 wt %; and the balance of iron (Fe), based on the total weight of the gray cast iron; and casting the molten steel by centrifugal casting, wherein the components of the molten steel satisfy the following Equation 1, 0.8≦Cu+1.5Cr+1.2Mo≦1.5  [Equation 1] where Cu, Cr, and Mo respectively mean the contents in wt % of Cu, Cr and Mo components.
 6. The method of claim 5, wherein after the centrifugal casting of the molten steel, the cylinder liner has a tensile strength of about 300 MPa or greater.
 7. The method of claim 5, wherein after the centrifugal casting of the molten steel, the cylinder liner has a fatigue strength of about 140 MPa or greater.
 8. A vehicle part that is manufactured from a gray casting iron of claim
 1. 